Code division multiple access systems, such as TD-SCDMA, are interference-restricted systems, in which the correlation of the users' spreading codes leads to MAI (multiple access interference) among the users. In the traditional receiving detection method, signal from other users are noise. However, with an increase in users, MAI becomes a large portion of the noise. As a result, the capacity of the system is restricted. Meanwhile, multi-path effects of the wireless channel will cause ISI (inter-symbol interference) among the codes.
A TD-SCDMA system can be an intra-frequency network, or inter-frequency network. When it is an inter-frequency network, multiple access of several cells is realized by frequency division. When it is intra-frequency, it is realized by compounded spreading codes—Walsh codes and scrambling codes.
In the case of an intra-frequency network, at the conjunct borderline of several cells (e.g. 2 or 3 cells), user equipment will receive interference either from its own cell, or from the other cells. The interference of its own cell can be fully utilized when joint-detected by the receiver and inflict little influence on the receptivity. However, if it is not counteracted, or joint-detected, the interference from the other cells (especially the adjacent cells) might disable the receptivity of the receiver. As such, the capacity and successful hand-over of the system might be decreased to a great extent.